This invention relates to optical distance measurement devices using image sensors, and more particularly to optical distance measurement devices for continuously measuring the distance to an object, such as an automobile, which exhibits a bilateral symmetry.
FIG. 5 is a diagram showing the principle of a conventional optical distance measurement device using image sensors, which is disclosed, for example, in Japanese Patent Publication (Kokoku) Nos. 63-38085 and 63-46363. As shown in FIG. 5, the optical distance measurement device includes a left and a right optical systems. The lenses 1 and 2 are separated by a base-line length L. Two images of an object 52 are formed upon the image sensors 3 and 4, respectively, positioned at the focal distance f of the respective lenses 1 and 2. A microcomputer 51 calculates the distance R to the object 52 from the shift a between the images formed upon the image sensors 3 and 4.
Namely, the microcomputer 51 superposes upon each other the two images formed upon the image sensors 3 and 4, successively changing the amount of the relative displacement of the two images. The microcomputer 51 thus determines the amount of shift a at which the best match between the two images is found. The distance R to the object 52 is determined based upon the principle of trigonometry by the following equation: EQU R=f.times.L/a (1)
The above conventional optical distance measurement device, however, has the following disadvantage. The images obtained by the left and right optical systems are compared with each other directly, to determine the distance R. Thus, if the optical axes of the two optical systems are displaced vertically, the agreement of the two images deteriorates, and hence the accuracy of measurement is reduced. Consequently, the vertical positions of axes of the two optical systems must accurately be adjusted. Further, the optical characteristics of the two systems are generally not exactly identical. The disagreement in the contrast and the brightness of the images resulting from the difference in the response characteristics of the auto-irises of the two systems: further reduces the agreement of the two images. This is particularly manifest when the brightness of the images changes abruptly. Thus the accuracy of the measurement is further reduced.